1. Field of the Invention
The present invention relates to a power source voltage controller having a replica circuit for monitoring critical path delay of a semiconductor circuit.
2. Description of the Related Art
Recent semiconductor circuits generally are reduced in power consumption by the method of lowering a power source voltage V.sub.DD.
This is because lowering the power source voltage is the most efficient for reducing the power consumption of a semiconductor circuit (LSI) since the AC component of the power consumption of the LSI is proportional to the square of the power source voltage.
From this viewpoint, the method has recently been reported of dynamically controlling the power source voltage to deal with the operating frequency of the LSI and the fluctuations in the process.
In a power source voltage controller using such a so-called variable power source voltage system, a replica circuit having the same power source voltage-delay characteristic as the critical path is provided, and the power source voltage is controlled while judging whether the replica circuit can normally operate.
An LSI using this variable power source voltage control system is extremely effective in reducing the power consumption of the LSI.
Summarizing the problems to be solved by the invention, since it is extremely difficult to reproduce the exact same effects due to layout and interconnection as an actual critical path in the replica circuit, it is necessary to operate the replica circuit by giving it a certain delay margin.
However, addition of a delay margin to the replica circuit leads to an increase of the operating power source voltage. An excessive margin results in less of an effect of the power source voltage control system.
Also, a power source voltage controller using a replica circuit uses a phase comparison system of comparing the phases of a clock signal propagated through the replica circuit and an original clock signal before propagation and controlling the power source voltage by an up or down signal in accordance with the result of the phase comparison in order that the delay of the replica circuit not exceed one cycle of the operating frequency.
In this phase comparison system, however, only information of whether the phase is advanced or delayed is obtained. Information of about how much phase difference there is cannot be obtained.
Accordingly, the steps by which the power source voltage step can be changed end up becoming fixed at a certain value no matter what the phase difference is.
The minimum step of the power source voltage becomes the amount of fluctuation of the power source voltage at the time of convergence, so the smaller the step width, the smaller the power source voltage fluctuation at the time of convergence, however, the time to the convergence becomes longer.
Accordingly, for more efficient use of the power source voltage system, a method for setting an optimal margin for a replica circuit and an apparatus realizing both a small power source voltage fluctuation and a short convergence time have been sought.